Conventionally, a Laval nozzle is used in an ejector cycle for increasing a flow speed of a fluid jetted from the Laval nozzle. When the flow speed of the fluid jetted from the Laval nozzle is increased, pumping effect of the ejector is enhanced by entrainment of the high-speed jet flow. The Laval nozzle has a throat section and a diffuser section downstream of the throat section. The throat section is the most reduced area in a fluid passage in the Laval nozzle. The diffuser section has a passage sectional area expanding toward downstream in the fluid passage from the throat section (e.g., JP-A-10-205898 and JP-A-5-312421).
In an ideal Laval nozzle, refrigerant (fluid) is throttled as flowing toward the throat section. The flow speed of the refrigerant is increased, and becomes mach 1 (critical condition) at the throat section. The refrigerant after passing through the throat section is boiled while expanding in the diffuser section, so that the flow speed of the refrigerant exceeds mach 1 in the diffuser section.
Further, a variable throttle valve can be provided at an upstream side of the nozzle for changing a refrigerant flow rate in the nozzle, so as to adapt to a change of an operating condition in the ejector cycle. Alternatively, an outlet passage area of the nozzle is changed, so as to adapt to a change of the operating condition in the ejector cycle.
The refrigerant flow rate passing through the nozzle substantially proportionally changes in accordance with a passage area of the throat section. Therefore, the diffuser section and the throat section must be precisely manufactured in the Laval nozzle. Thus, tolerance of the diffuser section and the throat section has to be strictly controlled. Especially, the tolerance of the diffuser section and the throat section has to be controlled within 100 μm in the Laval nozzle used in a home air conditioner or a vehicle air conditioner. Therefore, the Laval nozzle is difficult to be manufactured.
Besides, a flow characteristic of the Laval nozzle depends on a passage sectional area in the throat section. An appropriate expansion condition at the downstream side of the throat section depends on a passage sectional area of a nozzle outlet port. Therefore, if a refrigerating capacity of the ejector cycle is changed, i.e., specification of refrigerating capacity or a design pressure condition is changed, the Laval nozzle needs to be manufactured in accordance with the changing of the refrigerating capacity of the ejector cycle.
Therefore, when multiple kinds of ejector cycles are respectively constructed depending on various refrigerating capacities and various operating conditions, special nozzles need to be manufactured in accordance with each specification, such as the refrigerating capacity and the operating condition. That is, capital investment for constructing the ejector cycles is further needed, and additional manpower is also needed for manufacturing many kinds of the nozzles, so that manufacturing costs of ejector cycles, i.e., nozzles, are increased.
In general, the Laval nozzle is used in a refrigerator when fluctuation of a thermal load in the refrigerator is relatively small. In this case, the balance of the ejector cycle depends on a fixed characteristic of the Laval nozzle. Here, the Laval nozzle is initially designed based on a specific condition, such as a maximum thermal load condition. However, when the thermal load is changed, a pressure condition is also changed. In this case, the expansion condition of refrigerant is changed to be a deficient condition or an excessive condition.
As shown in FIG. 19A, when inlet pressure of the Laval nozzle is PH1, nozzle outlet pressure becomes an appropriate outlet pressure PL. In this case, refrigerant appropriately expands in the diffuser section of the Laval nozzle. However, when inlet pressure of the Laval nozzle becomes PH2, refrigerant does not sufficiently expand in the Laval nozzle. In this case, the nozzle outlet pressure does not decrease to the appropriate outlet pressure PL, and the nozzle efficiency is decreased. As shown in FIG. 19B, when outlet pressure of the Laval nozzle is PL3, pressure of refrigerant decreases from PH to PL3. In this case, refrigerant appropriately expands in the diffuser section of the Laval nozzle. However, when outlet pressure of the Laval nozzle becomes PL4, pressure of refrigerant does not decrease from PH to PL4. In this case, refrigerant does not sufficiently expand in the Laval nozzle, and the nozzle efficiency is decreased. Therefore, the nozzle efficiency decreases in the Laval nozzle, when either the inlet pressure or the outlet pressure is changed.
Alternatively, in a variable nozzle where the end of the needle valve is located on an upstream side of the throat section, the cross-sectional area of the fluid passage on the downstream side of the throat section cannot be changed. As shown in FIG. 20, as the refrigerant passage sectional area in the throat section (i.e., throttle area) is reduced, pressure at the throat section decreases. When the throttle area is small, as in the case where the pressure changes shown by □ and ♦, the pressure at the throat section once decreases, however, the pressure increases again toward the nozzle outlet. This pressure decreasing and increasing is caused due to an excessive expansion between the throat section and the nozzle outlet. In this case, refrigerant speed discharged from the nozzle cannot be increased up to the sound speed, and efficiency of the ejector decreases.
FIG. 21 is a graph showing a pressure distribution in the diffuser section of the nozzle and a dimensionless number D (i.e., passage sectional area in the diffuser section/passage sectional area in the throat section), when the throttle area (i.e., passage area in the throat section) is changed. In FIG. 21, carbon dioxide is used for the refrigerant, and temperature condition and pressure condition are constant in the nozzle inlet. The relationship shown in FIG. 21 can be applied to a flow over the sound speed, and cannot be applied to a flow condition in which pressure increases in the nozzle because a shock wave is generated due to an excessive expansion.
As shown in FIG. 21, even the cross-sectional area in the throat section (i.e., throttle area) is changed, the relationship between the pressure and the dimensionless number D does not largely change. Specifically, the relationships shown by the marks, such as ▴, ▪, □, ●, show similar characteristic, when the throttle area is changed. However, when the throttle area is reduced and the nozzle outlet area is not changed, the dimensionless number D (i.e., passage sectional area in the nozzle outlet/passage sectional area in throttle) becomes large at the nozzle outlet. In this case, if the refrigerant speed is over the sound speed and the refrigerant expands at the outlet of the nozzle, the pressure of the refrigerant becomes small according to the relationship in FIG. 21. Therefore, the refrigerant may excessively expand, because pressure of the refrigerant is decreased at the outlet of the nozzle. Accordingly, the nozzle efficiency of the conventional variable nozzle decreases.